1. Field of the Invention
The present invention relates to a heat-curable silicone resin sheet, which may be laminated on and bonded to the chip surface of an LED element, enabling conversion of the wavelength of blue light and ultraviolet light from the LED, and which has silicone resin layers of, at the very least, a phosphor-containing layer and a phosphor-free layer. The present invention also relates to a method of producing a light emitting device utilizing the resin sheet, and the light emitting device obtained by the same method.
2. Description of the Related Art
In the field of light emitting diodes (LED) the utilization of phosphors for wavelength conversion is known (Patent Document 1). Silicone resins are receiving attention as coating materials for the encapsulation and the protection of the LED element due to their excellent light resistance (Patent Document 2).
Generally, in white-colored LED elements, blue light is converted to a quasi-white light by dispersing phosphors in the vicinity of the chip in a method such as coating the LED chip with a silicone resin or an epoxy resin in which phosphors are dispersed. However, since color drift is likely to occur if the dispersion of the phosphors within the resin layer is not uniform or is uneven, it is necessary for the phosphors to be uniformly dispersed within the coating resin layer in order to produce a uniform white light. Consequently, a method in which a silicone resin composition containing phosphors is screen-printed has been investigated, for example. Furthermore, in another investigated method, after application of the composition to the chip followed by uniform dispersion of the phosphors in the vicinity of the chip through the precipitation to obtain a phosphor-dispersed layer, a transparent or semi-transparent protective layer is formed on the obtained layer. However, in this method, in addition to the insufficient stability of the quality of the obtained phosphor-dispersed layer and protective transparent layer, the complex production process is a problem. Furthermore, conventionally, the formation of the protective layer is performed by applying on the LED element a heat-curable silicone resin sheet containing the phosphors, curing thereof, and injecting a transparent resin into a mold. This method also has a problem of complex production process.
Furthermore, in an LED or the like, a high heat resistance and resistance to ultraviolet light is required for the resin layer coating the LED element. In addition, it will be convenient if the resin layer, in which the phosphors are uniformly dispersed, can be formed with conventional production apparatus.    Patent Document 1: Published Japanese Translation No. 2005-524737 of PCT International Publication.    Patent Document 2: Japanese Unexamined Patent Publication No. 2004-339482.    Patent Document 3: Japanese Unexamined Patent Publication No. 2009-094351.